MRI scanners use a strong, uniform static magnetic field, conventionally denoted B0, to cause protons (hydrogen nuclei) to be subject to a resonance at a particular radio frequency (RF), known as the Larmor frequency, which depends on the field strength: f0=γB0, γ=42.6 MHz/T. In that formula f0 is the Larmor frequency and B0 is the static magnetic field, with γ being a proportionality factor (gyromagnetic ratio). Near-field coils are used to excite protons at the Larmor frequency and to detect a signal from the oscillating protons. The main static field is typically aligned with the length of the scanner's bore. The RF excitations applied to excite the protons include a magnetic field conventionally denoted B1, typically transverse to the main static magnetic field and circularly polarized. The excitations typically have a bandwidth less than 100 kHz. These excitations at the Larmor frequency cause the spins of the protons, which initially are on average aligned with the main static field, to change orientation so that the spins on average point in a different direction. The spins precess about the main static field, causing the average orientation of the spins to also precess, producing a detectable signal. Typically, to encode position information in the signal to construct an image, gradient fields are applied that change the strength of the main static magnetic field depending on position. The RF excitations are typically applied in pulses of duration in the range of 100 μs-3 ms and with a repetition rate in the range of 5 ms to several seconds. Preferably, the amplitude of the excitations is uniform to achieve uniform image intensity and contrast, and high sensitivity (high Bimagnetic field per unit voltage excitation or power). For safety reasons, there are local and whole body specific absorption rate (SAR) constraints (IEC 60601-2-33).
Due to difficulties in producing a strong uniform magnetic field over a large volume, MRI scanners typically have narrow bores, which can lead to claustrophobia in patients. At the typical magnetic field strengths, the Larmor frequency for protons is sufficiently low and the bore sufficiently narrow that electromagnetic waves at the Larmor frequency cannot propagate through the bore. To produce the excitations, antennas known as a body coil or “birdcage” coil are provided within the bore. The birdcage coil 30 (an example shown in FIG. 1) is a resonant ladder network excited in CP (quadrature) mode. “Rungs” 32 each have a capacitor 34 to cause resonance. The static magnetic field B0 is indicated by arrow 36. Electrical connections to drive the birdcage coil are not shown. An array of separate receiving coils/loops (not shown) is used for maximum signal-to-noise ratio (SNR) in reception. The birdcage coil can also be used to receive, but the SNR will be low. The birdcage coil typically takes up significant space in the bore, increasing claustrophobia. Simply extending the birdcage too close to a conductive bore to reduce claustrophobia would create image currents on the bore, reducing efficiency. In FIG. 2, 40 shows an example magnetic field (uniform birdcage mode) represented by field lines 38, the magnetic field represented being the magnetic field B1 induced in the space within a conductive bore 40 at a point in time by a birdcage coil 30 within the bore.
Traditional body coils do not require typically subject-specific adjustments at lower B0 field strengths, but at high frequency they are highly sensitive to dielectric loading. These higher B0 fields and Larmor frequencies can be useful to improve signal- and contrast-to-noise ratios, allowing higher resolution. Body coils are also costly to build because they contain expensive components like high-voltage capacitors, and the use of a small number of localized elements requires tuning and balancing on the bench for optimal operation. Simply increasing the number of rungs to distribute the capacitance would increase the number of paths, and lead to a cluttered mode spectrum.
Travelling wave (TW) MRI has been one proposal to deal with the claustrophobia issue. Like a waveguide, the TW MRI bore has a cutoff frequency for propagating waves, and because of the size of bore required to accommodate the body of the patient, this cutoff frequency is in the order of several hundred MHz. For example, a typical MRI bore may be 58 cm in diameter and have a natural frequency cutoff of the TE11 mode of approximately 300 MHz. This natural cutoff frequency of the MRI bore prevents waves having a frequency below the natural cutoff frequency from propagating through the MRI bore. In a typical travelling wave MRI, a stronger magnetic field is used to increase the Larmor frequency for protons to above the cutoff for the bore. Antennas are then placed outside the bore to produce and detect excitations that can propagate through the bore. However, travelling wave MRI has developed a reputation for a lower image SNR than conventional MRI.
U.S. Pat. No. 9,529,062 describes a metamaterial liner for MRI for lowering a cutoff frequency of the MRI bore for use in travelling-wave magnetic resonance imaging. Metamaterials are periodic structures that can provide effective bulk permeability and permittivity responses beyond those found in nature. The lowering of the cutoff frequency allows travelling-wave MRI to be used in an MRI scanner with conventional field strength, so that a conventional MRI scanner can be retrofitted as a travelling-wave MRI scanner.
A metamaterial for lowering the cutoff frequency of a bore, using radial inductors and circumferential capacitors, was disclosed in Justin Pollock and Ashwin K. Iyer, “Experimental Verification of Below-Cutoff Propagation in Miniaturized Circular Waveguides Using Anisotropic ENNZ Metamaterial Liners”, IEEE Transactions On Microwave Theory and Techniques, vol. 64, no. 4, 2016.
The phrase “lowering the cutoff frequency” is used to mean that a new passband is introduced by the metamaterial liner in the otherwise below-cutoff frequency region. This passband is different than that above cutoff in an empty circular waveguide. The passband introduced by the metamaterial has a cutoff frequency below which propagation is permitted.